Handset mobile telephony or headphone playback require a loudspeaker to be placed close to (or inside) the listener's outer ear. This results in an acoustic coupling between the loudspeaker and the ear that strongly influences the acoustical pressure at the eardrum and consequently the perceived sound quality.
In the specific case of a handset mobile phone, the amount of acoustic coupling is not fixed and varies with the force manually applied to the mobile phone against the ear by the user. As the force applied becomes stronger, the sealing or fit of the loudspeaker to the ear improves which may result in reducing or lowering the acoustic leakage which in turn may improve the low frequency sound reproduction.
FIG. 1 illustrates the transfer function 100 of an existing mobile phone loudspeaker measured on a Head and Torso Simulator (HATS) for different application forces. The y-axis shows the relative sound pressure level in decibels and the x-axis shows the frequency ranging from 100 Hz to 10,000 Hz. The transfer function of the loudspeaker is illustrated for four conditions shown in lines 10, 12, 11 and 16 respectively. As the fit of the loudspeaker to the listener's ear becomes tighter, the level of the frequency response increases at low frequencies. Graph line 16 for example shows the situation where the fit of the loudspeaker to a listener's ear is relatively loose and line 10 of the frequency response shows the condition where the loudspeaker is a tight fit.
The variation of acoustic leakage while a mobile phone is in use can have an adverse effect on the sound quality perceived by the user.
Two main techniques exist to minimize the impact of leakage on the acoustic response of the speaker. Firstly, leak tolerant acoustical designs as described for example in U.S. Pat. No. 6,658,110 B1 and U.S. Pat. No. 6,788,798 B1. Leak tolerant acoustical designs may minimize the effect of leakage on the total acoustic load of the loudspeaker by introducing controlled acoustical paths. However they require a complex loudspeaker housing to reduce the adverse impact of acoustic leakage. Secondly a dedicated sensor in close proximity to the loudspeaker as described in U.S. Pat. No. 6,639,987B2 may be used to detect the amount of acoustic leakage while the mobile phone is in use and then equalize the audio signal accordingly.